MAX1708 Datasheet (PDF) - Maxim Integrated Products

7/24/2019 MAX1708 Datasheet (PDF) - Maxim Integrated Products

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General Description

The MAX1708 sets a new standard of space savings forhigh-power, step-up DC-DC conversion. It delivers upto 10W at a fixed (3.3V or 5V) or adjustable (2.5V to5.5V) output, using an on-chip power MOSFET from a+0.7V to +5V supply.

Fixed-frequency PWM operation ensures that theswitching noise spectrum is constrained to the 600kHzfundamental and its harmonics, allowing easy postfilter-ing for noise reduction. External clock synchronizationcapability allows for even tighter noise spectrum con-trol. Quiescent power consumption is less than 1mW toextend operating time in battery-powered systems.

Two control inputs (ONA, ONB) allow simple push-on,push-off control through a single momentary push-but-

ton switch, as well as conventional on/off logic control.The MAX1708 also features programmable soft-startand current limit for design flexibility and optimum per-formance with batteries. The maximum RMS switch cur-rent rating is 5A. For a device with a higher (10A)switch current rating, refer to the MAX1709 data sheet.

________________________Applications

Routers, Servers, Workstations, Card Racks

Local 2.5V to 3.3V or 5V Conversion

Local 3.3V to 5V Conversion

3.6V or 5V RF PAs in Communications Handsets

Features

On-Chip 5A Power MOSFET

5V, 2A Output from a 3.3V Input

Fixed 3.3V or 5V Output Voltage or

Adjustable (2.5V to 5.5V)

Input Voltage Range Down to 0.7V

Low Power Consumption

1mW Quiescent Power

1A Current in Shutdown Mode

Low-Noise, Constant Frequency Operation

(600kHz)

Synchronizable Switching Frequency(350kHz to 1000kHz)

Small QSOP Package

MAX1708

High-Frequency, High-Power, Low-Noise,Step-Up DC-DC Converter

________________________________________________________________ Maxim Integrated Products 1

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

ONB CLK

3.3/5

PGND

PGND

PGND

FB

OUT

GND

TOP VIEW

MAX1708

QSOP

ONA

LX

GND

LX

LX

SS/LIM

REF

Pin Configuration

ONA

REF

SS/LIM

CLK

OUTPUT3.3V, 5V,OR ADJUP TO 2A

OUT

GND

LX

2.2H

OFF ON

INPUT1V TO 5V

SYNCOR

INTERNAL

MAX1708

Typical Operating Circuit

19-2068; Rev 0; 7/01

Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX1708EEE -40C to +85C 16 QSOP

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at1-888-629-4642, or visit Maxims website at www.maxim-ic.com.


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MAX1708


2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS(VOUT = VCLK = +3.6V, ONA = ONB= FB = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)

Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.

ONA, ONB, OUT, SS/LIM, 3.3/5 to GND ...............-0.3V to +6.0VLX to PGND ...........................................................-0.3V to +6.0VFB, CLK, REF to GND.............................. -0.3V to (VOUT + 0.3V)PGND to GND .......................................................-0.3V to +0.3VContinuous Power Dissipation (TA = +70C)

16-Pin QSOP (derate 8.30mW/C above +70C). .......667mW

Operating Temperature Range ...........................-40C to +85CJunction Temperature......................................................+150CStorage Temperature Range............ .................-65C to +150CLead Temperature (soldering, 10s).. ...............................+300C

PARAMETER CONDITIONS MIN TYP MAX UNITS

3.3/5 = GND, ISW= 0.5A 3.26 3.34 3.42Output Voltage VFB< 0.1V (Note 1)

3.3/5 = OUT, ISW= 0.5A 4.90 5.05 5.20V

Load Regulation Measured between 0.5A < ISW< 1.5A (Note 2) -0.40 -0.60 %/A

FB Regulation Voltage (VFB) ISW= 0.5A 1.215 1.240 1.265 V

FB Input Current VFB = +1.5V 1 200 nA

Output Voltage Adjust Range 2.5 5.5 V

Output Undervoltage Lockout

(Note 3)Rising and falling 2.0 2.3 V

Frequency in Startup Mode VOUT=1.5V 40 400 kHz

Minimum Startup Voltage IOUT< 1mA, TA= +25C (Note 4) 0.9 1.1 V

Minimum Operating Voltage (Note 5) 0.7 V

Soft-Start Pin Current VSS/LIM

= 1V 3.2 4 5.0 A

OUT Supply Current VFB= 1.5V (Note 6) 200 300 A

OUT Leakage Current In

ShutdownVONB= 3.6V 0.1 2 A

LX Leakage Current VLX= VONB= VOUT= +5.5V 1 25 A

N-Channel Switch

On-Resistance30 80 m

SS/LIM = open 4.5 5.3 7.0N-Channel Current Limit

SS/LIM = 150kto GND 1.80 3.00 3.85A

RMS Switch Current 5 ARMS

Reference Voltage IREF= 0 1.245 1.260 1.275 V

Reference Load Regulation -1A IREF50A 4 10 mV

Reference Supply Rejection +2.5V VOUT+5.5V 0.2 5 mV

ONA, ONB, 3.3/5, 1.2V < VOUT< 5.5V 0.2 VOUTInput Low Level (Note 7)

CLK, 2.7V < VOUT < 5.5V 0.2 VOUTV


3/13

MAX1708


_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VOUT = VCLK = +3.6V, ONA = ONB= FB = GND, TA= 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)

ELECTRICAL CHARACTERISTICS(VOUT = VCLK = +3.6V, ONA = ONB= FB = GND, TA= -40C to +85C, unless otherwise noted.) (Note 9)

PARAMETER CONDITIONS MIN TYP MAX UNITS

ONA, ONB, 3.3/5, 1.2V < VOUT


4/13

Note 1: Output voltage is specified at 0.5A switch current ISW, which is equivalent to approximately 0.5A (VIN/ VOUT) of load cur-

rent.

Note 2: Load regulation is measured by forcing specified switch current and straight-line calculation of change in output voltage in

external feedback mode. Note that the equivalent load current is approximately ISW (VIN/ VOUT).

Note 3: Until undervoltage lockout is reached, the device remains in startup mode. Do not apply full load until this voltage is

reached.

Note 4: Startup is tested with Figure 1s circuit. Output current is measured when both the input and output voltages are applied.

Note 5: Minimum operating voltage. The MAX1708 is bootstrapped and will operate down to a 0.7V input once started.

Note 6: Supply current is measured from the output voltage (+3.3V) to the OUT pin. This correlates directly with actual input supply

current but is reduced in value according to the step-up ratio and efficiency.

Note 7: ONA and ONB inputs have approximately 0.15V hysteresis.

Note 8: Guaranteed by design, not production tested.

Note 9: Specifications to -40C are guaranteed by design, not production tested.

MAX1708


4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(VOUT = VCLK = +3.6V, ONA = ONB= FB = GND, TA= -40C to +85C, unless otherwise noted.) (Note 9)

PARAMETER CONDITIONS MIN MAX UNITS

ONA, ONB, 3.3/5, 1.2V < VOUT< 5.5V 0.2 VOUTInput Low Level (Note 7)

CLK, 2.7V < VOUT< 5.5V 0.2 VOUTV

ONA, ONB, 3.3/5, 1.2V < VOUT< 5.5V 0.8 VOUTInput High Level

CLK, 2.7V < VOUT< 5.5V 0.8 VOUTV

Logic Input Current ONA, ONB, CLK, 3.3/5 = 0, 5.5V -1 1 A

Internal Oscillator Frequency 500 700 kHz

Maximum Duty Cycle 80 95 %

External Clock Frequency Range 350 1000 kHz

CLK Pulse Width (Note 8) 100 nsCLK Rise/Fall Time (Note 8) 50 ns


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MAX1708


_______________________________________________________________________________________ 5

Typical Operating Characteristics(Circuit of Figure 1, TA = +25C, unless otherwise noted.)

100

00.1 1 1000 10,000

EFFICIENCY vs. OUTPUT CURRENT

MAX1708toc01a

OUTPUT CURRENT (mA)

EFFICIENCY(%)

10 100

20

40

60

80VIN= 3.3V

VIN= 2.5V

VOUT= 5V

2.0

-2.00.1 1 1000 10,000

LOAD REGULATION(VIN= 3.3V, VOUT= 5V)

-1.5

0.5

1.0

1.5

MAX1708toc03a

OUTPUT CURRENT (mA)

VOUT

REGULATION(%)

10 100

-1.0

-0.5

0

PLOT NORMALIZED TO IOUT= 500mA

2.0

-2.00.1 1 1000 10,000

LOAD REGULATION(VIN= 2.5V, VOUT= 3.3V)

-1.5

0.5

1.0

1.5

MAX1708toc03b

OUTPUT CURRENT (mA)

VOUT

REGULATION(%)

10 100

-1.0

-0.5

0

PLOT NORMALIZED TO IOUT= 500mA

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1.50 1.75 2.00 2.25 2.50 2.75 3.00

LINE REGULATION(VOUT= 3.3V)

MAX1708toc04b

INPUT VOLTAGE (V)

VOUTREGULAT

ION(%)

PLOT NORMALIZED TO VIN= 2.5V

IOUT= 500mA

IOUT= 1A

0

5

15

10

20

25

0 21 3 4 5 6

NO LOAD INPUT CURRENTvs. INPUT VOLTAGE

MAX1708toc05a

INPUT VOLTAGE (V)

INPUTCURRENT(mA) VOUT= 5V, VININCREASING

VOUT= 3.3V, VININCREASING

0

20

10

40

30

60

50

70

0 2 31 4 5 6

NO LOAD INPUT CURRENTvs. INPUT VOLTAGE

MAX1708toc05b

INPUT VOLTAGE (V)

INPUTCURRE

NT(mA)

VOUT= 3.3V, VINDECREASING

VOUT= 5V, VINDECREASING

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

2.00 2.502.25 2.75 3.00 3.25 3.50 3.75 4.00

LINE REGULATION(VOUT= 5V)

MAX1708toc04a

INPUT VOLTAGE (V)

VOUT

REGULATION(%)

PLOT NORMALIZED TO VIN= 3.3V

IOUT= 500mAIOUT= 1A

100

00.1 1 1000 10,000

EFFICIENCY vs. OUTPUT CURRENT

MAX1708toc01b

OUTPUT CURRENT (mA)

EFFICIENCY(%)

10 100

20

40

60

80

VIN= 1.2V

VIN= 2.5V

VOUT= 3.3V86

87

88

89

90

EFFICIENCY vs. SWITCHING FREQUENCY

MAX1708toc02

SWITCHING FREQUENCY (kHz)

EFFICIENCY(%)

350 650 750450 550 850 950

VIN= 3.3V, VOUT= 5V, IOUT=1A


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MAX1708


6 _______________________________________________________________________________________

1000 10,0000

0.2

0.4

0.6

0.8

1.2

1.0

1.6

1.4

1.8

1 10 100

STARTUP VOLTAGE vs. LOAD CURRENT(VOUT= 5V)

MAX1708toc06a

LOAD CURRENT (mA)

STARTUPVOLTAGE(V)

TA= -40C

TA= +25C

TA= +85C

2.5

2.0

1.5

1.0

0.5

0

1 100 100010 10,000

STARTUP VOLTAGE vs. LOAD CURRENT(VOUT= 3.3V)

MAX1708toc06b

LOAD CURRENT (mA)

STARTUPVOLTAGE(V)

TA= -40C

TA= +25C

TA= +85C

2

1

0

-1

-2-40 10-15 35 60 85

SWITCHING FREQUENCYvs. TEMPERATURE

MAX1708toc07

TEMPERATURE (C)

FREQUENCYCHANGE(%)

VOUT= 3.3V

PLOT NORMALIZED TO 25C

4000

00.1 1 10

NOISE vs. FREQUENCY

MAX1708toc08

FREQUENCY (MHz)

NOISE(V)

500

1000

2000

1500

3000

3500

2500

RESOLUTION = 1kHz

500 250200150100 300

SWITCH CURRENT LIMITvs. SS/LIM RESISTANCE

MAX1708toc09

SS/LIM RESISTANCE (k)

SWITCHCURRENTLIMIT(A)

1

2

0

4

5

3

6.0

5.5

5.0

4.5

4.0-40 10-15 35 60 85

SWITCH CURRENT LIMITvs. TEMPERATURE

MAX1708toc10

TEMPERATURE (C)

SWITCH

CURRENTLIMIT(A)

VIN= 3.3V, VOUT= 5V

1s/div

HEAVY SWITCHING WAVEFORM

VOUTAC-COUPLED

50mV/div

5V

4A

2A

5V

0

0

VLX5V/div

IL2A/div

MAX1708 toc11

100s/div

LINE TRANSIENT RESPONSE

VOUTAC-COUPLED

50mV/div

3.5V

3V

5V

VIN500mV/div

MAX1708 toc12

Typical Operating Characteristics (continued)(Circuit of Figure 1, TA = +25C, unless otherwise noted.)


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MAX1708


_______________________________________________________________________________________ 7

Typical Operating Characteristics (continued)(Circuit of Figure 1, TA = +25C, unless otherwise noted.)

40s/div

LOAD TRANSIENT RESPONSE

IOUT1A/div

VOUTAC-COUPLED

50mV/div

4A

5V

2A

1A

2A

0

0

0

ISW2A/div

MAX1708 toc13

2ms/div

STARTUP WITHOUT SOFT-START(CSS= 0)

VOUT2V/div

IIN1A/div

5V

2A

1A

0

4V

0

2V

VONA5V/div

MAX1708 toc14

RL= 5

2ms/div

STARTUP WITH SOFT-START(CSS= 0.01F)

VOUT2V/div

IIN1A/div

5V

1A

0

2A

4V

2V

0

0

VONA5V/div

MAX1708 toc15

RL= 5

2ms/div

STARTUP WITH SOFT-START(CSS= 0.1F)

VOUT2V/div

IIN1A/div

5V

1A

0

4V

2V

0

0

VONA5V/div

MAX1708 toc16

RL= 5


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MAX1708


8 _______________________________________________________________________________________

Pin Description

PIN NAME FUNCTION

1 ONB Shutdown Input. When ONB= high andONA = low, the device turns off (Table 1).

2 ONA On-Control Input. When ONA = high orONB= low, the device turns on (Table 1).

3, 4, 5 LXDrain of N-Channel Power Switch. Connect pins 3, 4, and 5 together with wide traces. Connect

an external Schottky diode from LX to VOUT. (Figure 1)

6, 9 GND Ground

7 SS/LIM

Soft-Start and/or Current-Limit Input. Connect a capacitor from SS/LIM to GND to control the

rate at which the device reaches current limit (soft-start). To reduce the current limit from the

preset values, connect a resistor from SS/LIM to GND (see Design Procedure). During

shutdown, SS/LIM is internally pulled to GND to discharge the soft-start capacitor.

8 REFVoltage Reference Output. Bypass with a 0.22F capacitor to GND. Maximum REF load is

50A.

10 OUT

Output Voltage Sense Input. The device is powered from OUT. Bypass with a 0.1F capacitor

to PGND with less than 5mm trace length. Connect a 2series resistor from the output filtercapacitor (0.1F) to OUT (Figure 1).

11 FB

DC-DC Converter Feedback Input. Connect FB to GND for internally set output voltage (see

3.3/5 pin description). Connect a resistor-divider from the output to set the output voltage in the

+2.5V to +5.5V range. FB regulates to +1.24V (Figure 4).

12, 13, 14 PGNDPower Ground, Source of N-Channel Power MOSFET Switch. Connect pins 12, 13, and 14

together with wide traces.

15 3.3/5

Output Voltage Selection Input. When FB is connected to GND, the regulator uses internal

feedback to set the output voltage. 3.3/5 = low sets output to 3.3V; 3.3/5 = high sets output to

5V. If an external divider is used at FB, connect 3.3/5 to ground.

16 CLK Clock Input for the DC-DC Converter. Connect to OUT for internal oscillator. Drive CLK withan external clock for external synchronization.


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MAX1708


_______________________________________________________________________________________ 9

_______________Detailed Description

The MAX1708 step-up converter offers high efficiencyand high integration for high-power applications. Itoperates with an input voltage as low as 0.7V and issuitable for single- to 3-cell battery inputs, as well as2.5V or 3.3V regulated supply inputs. The output volt-age is preset to +3.3V or +5.0V or can be adjusted withexternal resistors for voltages between +2.5V to +5.5V.

The MAX1708 internal N-channel MOSFET switch israted for 5A (RMS value) and can deliver loads to 2A,depending on input and output voltages. For flexibility,the current limit and soft-start rate are independentlyprogrammable.

A 600kHz switching frequency allows for a small induc-tor to be used. The switching frequency is also syn-

chronizable to an external clock ranging from 350kHzto 1MHz.

ONA, ONBThe logic levels at ONA and ONB turn the MAX1708 onor off. When ONA = 1 or ONB = 0, the device is on.When ONA = 0 and ONB= 1, the device is off (Table1). Logic high on-control can be implemented by con-necting ONB high and using ONA for shutdown.

Implement inverted single-line on/off control by ground-ing ONA and toggling ONB. Implement momentary

pushbutton on/off as described in the ApplicationsInformation section. Both inputs have approximately0.15V of hysteresis.

Switching FrequencyThe MAX1708 switches at the fixed-frequency internaloscillator rate (600kHz) or can be synchronized to anexternal clock. Connect CLK to OUT for internal clockoperation. Apply a clock signal to CLK to synchronizeto an external clock. The MAX1708 will synchronize to anew external clock rate in two cycles and will takeapproximately 40s to revert to its internal clock fre-quency once the external clock pulses stop and CLK isdriven high. Table 2 summarizes oscillator operation.

OperationThe MAX1708 switches at a constant frequency(600kHz) and modulates the MOSFET switch pulsewidth to control the power transferred per cycle andregulate the voltage across the load. In low-noise appli-cations, the fundamental and the harmonics generatedby the fixed switching frequency are easily filtered out.Figure 2 shows the simplified functional diagram for theMAX1708. Figure 3 shows the simplified PWM con-

Figure 1. Standard Operating Circuit

KEEP TRACESSHORT AND WIDE

ON/OFFCONTROL

MAX1708

L12.2H

C1150F

VIN

GNDR1

C3

C40.22F

GND

REF

SS/LIM

PGND PGND PGND

C50.1F

FB

GND

OUT

3.3/5

CLK

D1

LX LX LXR22

C2150F

VOUT+5V

GND

ONA

ONB

KEEP TRACESSHORT AND WIDE


10/13

MAX1708


10 ______________________________________________________________________________________

troller functional diagram. The MAX1708 enters syn-

chronized current-mode PWM when a clock signal(350kHz < fCLK < 1MHz) is applied to CLK. For wire-less or noise-sensitive applications, this ensures thatswitching harmonics are predictable and kept outsidethe IF frequency band(s). High-frequency operationpermits low-magnitude output ripple voltage and mini-mum inductor and filter capacitor size. Switching loss-es will increase at higher frequencies (see MAX1708 ICPower Dissipation).

Setting the Output VoltageThe MAX1708 features Dual Mode operation. WhenFB is connected to ground, the MAX1708 generates afixed output voltage of either +3.3V or +5V, dependingon the logic applied to the 3.3/5 input (Figure 1). The

output can be configured for other voltages, using twoexternal resistors as shown in Figure 4. To set the out-put voltage externally, choose an R3 value that is largeenough to minimize load at the output but small enoughto minimize errors due to leakage and the time constantto FB. A value of R4 50k is required

where VFB = 1.24V.

Soft-Start/Current Limit Adjustment(SS/LIM)

The soft-start pin allows the soft-start time to be adjust-ed by connecting a capacitor from SS/LIM to GND.Select capacitor C3 (see Figure 1):

tSS = 4ms + [110 C3 (in F)]

where tSS is the time (in milliseconds) it takes output toreach its final value.

To improve efficiency or reduce inductor size atreduced load currents, the current limit can be reduced

from its nominal value (see Electrical Characteristics).A resistor (R1 in Figure 1) between SS/LIM and groundreduces the current limit as follows:

where I1 is the desired current limit in amperes and R1 312k. ILIM = 5A, if R1 is omitted.

R kI

ILIM1 312 1=

R RV

V

OUT

FB

3 4 1=

ONA ONB MAX1708

0 0 On

0 1 Off

1 0 On

1 1 On

Figure 2. Simplified Functional Diagram

Table 2. Selecting Switching FrequencyTable 1. On/Off Logic Control

CLK MODE

0 Not allowed

1 PWM

External clock

(350kHz1000kHz) Synchronized PWM

N

PWMCONTROLLER

STARTUPOSCILLATOR

OSC

UNDERVOLTAGE LOCKOUT

FB

EN

DEN Q

600kHzOSCILLATOR

ENREFERENCE

ON RDY

DUAL MODEFB

LX

PGND

1.26V

2.15V

OUT

ONA

ONB

REF

CLK

FB

3.3/5

GND

IC POWER MAX1708

OUT

Dual Mode is a trademark of Maxim Integrated Products.


11/13

MAX1708


______________________________________________________________________________________ 11

__________________Design Procedure

Inductor Selection (L1)The MAX1708s high switching frequency allows theuse of a small-size inductor. Use a 2.2H inductor for600kHz operation. If the MAX1708 is synchronized at adifferent frequency, scale the inductor value with theinverse of frequency (L1 = 2.2H 600kHz / fSYNC).The PWM design tolerates inductor values within 25%of this calculated value, so choose the closest standardinductor value. For example, use 3.3H for 350kHz and

1.5H for 1MHz).Inductors with a ferrite core or equivalent are recom-mended; powder iron cores are not recommended foruse at high switching frequencies. Ensure the induc-tors saturation rating (the current at which the corebegins to saturate and inductance falls) exceeds the

internal current limit. Note that this current may bereduced through SS/LIM if less than the MAX1708s fullload current is needed (see Electrical Characteristicsfor ratings). For highest efficiency, use a coil with lowDC resistance, preferably under 20m. To minimizeradiated noise, use a toroid, pot core, or shieldedinductor. See Tables 3 and 4 for a list of recommendedcomponents and component suppliers. To calculate

the maximum output current (in amperes), use the fol-lowing equation:

where:VIN = input voltageVD = forward voltage drop of the Schottky diode

at ILIMVOUT = output voltageD' = (VIN) / (VOUT + VD), neglecting switch voltage

dropf = switching frequency

L1 = inductor valueILIM = minimum value of switch current limit from

Electrical Characteristicsor set by R1 ofFigure 1.

I D I DV V V

LOUT MAX LIM

OUT D IN( ) ' '=

+

2 1

SUPPLIER PHONE FAX

Central 631-435-1110 631-435-1824

Coilcraft 847-639-6400 847-639-1489

Coiltronics 561-241-7876 561-241-9339

Motorola 602-303-5454 602-994-6430

Panasonic 714-373-7939 714-373-7183

Table 4. Component Suppliers

Table 3. Component Selection Guide

PRODUCTION INDUCTORS CAPACITORS DIODES

Coiltronics UP2B-2R2 Sanyo 6TPC100M Motorola MBRD1035CTLSurface mount

Coilcraft DO3316P-222HC Panasonic EEFUE0J151R Central CMSH5-20

Figure 3. Simplified PWM Controller Functional Diagram

NR

S

Q

LX

PGND

11m

REFSLOPECOMP

FB

SS/LIM12.5

OSCILLATOR

(LIMITED TO 100mV)

Figure 4. Adjustable Output Voltage

R4

R3KEEP SHORT

FB

VIN

VOUTLX

MAX1708


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MAX1708


12 ______________________________________________________________________________________

Diode Selection (D1)The MAX1708s high switching frequency demands ahigh-speed rectifier. Use Schottky diodes (Table 3).The diodes current rating must exceed the maximumload current, and its breakdown voltage must exceedVOUT. The diode must be placed within 10mm of theLX switching node and the output filter capacitor. Thediode also must be able to dissipate the power calcu-lated by the following equation:

PDIODE = IOUT VD

where IOUT is the average load current and VD is the

diode forward voltage at the peak switch current.

Capacitor Selection

Input Bypass Capacitor (C1)A 150F, low-ESR input capacitor will reduce peak cur-rents and reflected noise due to inductor current ripple.Lower ESR allows for lower input ripple current, butcombined ESR values up to 100m are acceptable.Smaller ceramic capacitors may also be used for lightloads or in applications that can tolerate higher inputcurrent ripple.

Output Filter Capacitor (C2)The output filter capacitor ESR must be kept under30m for stable operation. Polymer capacitors of150F (Panasonic EEFUE0J151R) typically exhibit10m of ESR. This translates to approximately 35mV ofoutput ripple at 3.5A switch current. Bypass theMAX1708 IC supply input (OUT) with a 0.1F ceramiccapacitor to GND and a 2 series resistor (R2, asshown in Figure 1).

MAX1708 IC Power Dissipation

The major components of MAX1708 dissipated powerare switch conductance loss (PSW), capacitive loss(PCAP), and switch transition loss (PTRAN). Numericalexamples provided in brackets ({ }) correspond to thefollowing condition:

{VIN = 3.3V, VOUT = 5V, VD = 0.5V, IOUT = 2A}

An important parameter to compute the power dissipat-ed in the MAX1708 is the approximate peak switch cur-rent (ISW):

PD = PSW + PCAP + PTRAN {0.472W}

PSW = (1 - D') ISW2 RSW {0.353W}

PCAP = (CDIO + CDSW + CGSW) (VOUT + VD)2f {0.045W}

PTRAN = (VOUT + VD) ISW tSW f / 3 {0.073W}

where:

RSW = switch resistance {80m}CDIO = catch-diode capacitance {500pF}

CDSW = switch drain capacitance {1250pF}

CGSW = switch gate capacitance {750pF}

f = switching frequency {600kHz}tSW = switch turn-on or turn-off time {20ns}

Applications Information

Using a Momentary On/Off SwitchA momentary pushbutton switch can be used to turnthe MAX1708 on and off. As shown in Figure 5, whenONA is pulled low and ONB is pulled high, the deviceis off. When the momentary switch is pressed, ONB ispulled low and the regulator turns on. The switchshould be on long enough for the microcontroller to exitreset. The controller issues a logic high to ONA, which

guarantees that the device will stay on regardless ofthe subsequent switch state. To turn the regulator off,depress the switch long enough for the controller toread the switch status and pull ONA low. When theswitch is released, ONB pulls high and the regulatorturns off.

Layout ConsiderationsDue to high inductor current levels and fast switchingwaveforms, proper PC board layout is essential. Protect

II

DA

DV

V V

SWOUT

IN

OUT D

'

{ . }

' { . }

=

= +

3 33

0 6

C

270k

ONB

ONA

0.1F

270k

ON/OFF

MAX1708

VDD

I/O

I/O

Figure 5. Momentary Pushbutton On-Off Switch


13/13

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses areimplied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

sensitive analog grounds by using a star ground config-

uration. Connect PGND, the input bypass capacitorground lead, and the output filter capacitor ground leadto a single point (star ground configuration). In addition,minimize trace lengths to reduce stray capacitance andtrace resistance, especially from the LX pins to the catchdiode (D1) and output capacitor (C2) to PGND pins. If anexternal resistor-divider is used to set the output voltage(Figure 4), the trace from FB to the resistors must beextremely short and must be shielded from switchingsignals, such as CLK or LX. To optimize package powerdissipation and minimize device heating under heavyloads, expand PC trace area connected to the threePGND pins as much as the layout can allow. This is best

accomplished with a large PGND plane on the surface of

the board. Also note that outer-layer ground plane areabeneath the device provides little heat-sinking benefit. Ifan outer-layer ground plane is not feasible, the PGNDpins should be connected to the inner-layer groundplane with multiple vias (at least three vias per pin is rec-ommended). Since the purpose of these vias is to opti-mize thermal conductivity to the inner ground plane, besure that the vias have no gaps in their connections tothe ground plane. Refer to a layout example in theMAX1708EVKIT data sheet.

Package Information

___________________Chip InformationTRANSISTOR COUNT: 1112

SUBSTRATE: GND

PROCESS: BiCMOS

QSOP.E

PS


MAX1708

MAX1708 Datasheet (PDF) - Maxim Integrated Products

Documents

Transcript of MAX1708 Datasheet (PDF) - Maxim Integrated Products